This invention relates to a method for preparing a particular class of organotin compounds. This invention further relates to a method for preparing organotin derivatives of mercaptoalcohol esters which offers advantages with respect to known methods for preparing this class of organotin compounds.
U.S. Pat. No. 2,870,182 discloses compounds of the general formula R.sub.n SnA.sub.4-n wherein R represents one of a specified group of hydrocarbon radicals, n is 1, 2 or 3 and A represents the residue obtained following removal of the hydrogen atom from the --SH group of a mercaptoalcohol ester. The patent further discloses that compounds corresponding to the foregoing formula can be prepared by first reacting the mercaptoalcohol with a carboxylic acid in the presence of a suitable esterification catalyst and subsequently reacting the resultant ester with an organotin halide, oxide or with an organostannoic acid. This preparative method is less than desirable for a number of reasons. Firstly, formation of the mercaptoalcohol ester is an equilibrium reaction which almost always requires an acidic catalyst and removal of water during the reaction to obtain a useful yield of the desired product within a reasonable length of time. The acid catalyst may promote a number of undesirable side reactions, including polymerization of the mercaptoalcohol. The polymer may contain end groups that will subsequently react with the organotin compound, however the product is not nearly so effective as the desired monomeric ester derivative in a number of applications, including stabilization of vinyl chloride polymers. A second undesirable feature of the aforementioned prior art method is that removal of water is necessary during preparation of the ester and during reaction of the ester with the organotin compound. Removal of water requires heating, which not only increases processing costs due to the additional energy input, but can result in larger amounts of by-products from side reactions. In addition, a considerable amount of mercaptoalcohol is often removed together with the water, thereby necessitating use of a stoichiometric excess of this reagent. It has now been found that the disadvantages inherent in the prior art method can be avoided if the mercaptoalcohol is first reacted with the organotin compound and then esterified. Reactions of organotin halides and oxides with both mercaptans and alcohols are reported in the chemical literature. One would therefore expect to obtain a mixture of at least two products, one of which contains tin-oxygen bonds and a second compound containing tin-sulfur bonds. Surprisingly, under the conditions disclosed hereinafter only the mercaptide (--SH) portion of the mercaptoalcohol reacts with the organotin compound. The hydroxyl portion of the molecule remains available for subsequent esterification with a carboxylic acid or ester thereof.